In the past, in an internal combustion engine designed to control an air-fuel ratio to a target air-fuel ratio, it is known to arrange a limit current type air-fuel ratio sensor generating a limit current corresponding to the air-fuel ratio in an engine exhaust passage. In such an internal combustion engine, the amount of fuel fed to a combustion chamber is controlled by feedback by the air-fuel ratio sensor so that the air-fuel ratio becomes the target air-fuel ratio. In this regard, in this air-fuel ratio sensor, an element thereof is sometimes cracked, which results in the outer surface of the sensor element and the internal space of the sensor element being communicated. If an element cracking, the air-fuel ratio sensor can no longer generate a suitable output corresponding to the air-fuel ratio. As a result, the air-fuel ratio can no longer be accurately controlled by feedback to the target air-fuel ratio.
Therefore, an abnormality diagnosis system for detecting a crack of element of an air-fuel ratio sensor has been known in the past (for example, PTL 1). According to PTL 1, usually a voltage applied to an air-fuel ratio sensor is set to a center of a limit current region. If the sensor element of the air-fuel ratio sensor has cracked or if the platinum on the electrodes has shrunken, it is believed that the voltage applied to the air-fuel ratio sensor will deviate to the high voltage side from the center of the limit current region. Therefore, in the system described in this PTL 1, when the voltage applied to the air-fuel ratio sensor deviates to the high voltage side or low voltage side from the center part of the limit current region, it is judged that the sensor element of the air-fuel ratio sensor has cracked or the platinum on the electrodes has shrunken.